Recently, commercialization of driving safety support systems utilizing an on-vehicle communication device which performs inter-vehicle wireless communication is being considered. In the on-vehicle communication device, an information exchange application which transmits/receives information of own vehicle per cycle between other vehicles and itself is typically used. Moreover, in the inter-vehicle wireless communication system, a carrier sense multiple access (CSMA) system has been conventionally used as an access system such that each vehicle transmits information by itself.
When an information exchange application is used in the CSMA system, in a case where the number of vehicles positioned in a communication area increases, communication traffic increases to exceed a communication capacity. Further, in the CSMA system, vehicles transmit information by themselves, and thus a collision of information occurs when the timings at which information is transmitted are the same, which makes it impossible to receive information normally. Therefore, it is conceivable that congestion in which reliability of communication deteriorates may occur, and information through inter-vehicle wireless communication is not transmitted without fail, and accordingly safety support service cannot be provided.
Patent Document 1 discloses the following technology for preventing congestion from occurring in an inter-vehicle wireless communication system. That is, Patent Document 1 discloses the method of controlling a transmission cycle of own vehicle based on a dangerous situation of own vehicle and a traffic amount of channels, to thereby avoid congestion.
Further, in order to reduce congestion in an inter-vehicle wireless communication system, Patent Document 2 discloses the technique of reducing transmission output of inter-vehicle wireless communication as a degree of congestion increases.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-209333
Patent Document 2: Japanese Patent Application Laid-Open No. 2005-039665